The present invention relates to a radio communication apparatus and a communication method, and more particularly to a radio communication apparatus and a communication method for forming a frame with a plurality of slots and inserting a guard interval into each slot along with data for a mobile station to perform communication.
An example of a radio communication system using a guard interval is an orthogonal frequency division multiplexing (OFDM) radio communication system. An OFDM radio communication system is a system which multiplies each of a plurality of orthogonal frequencies (sub-carriers) by a symbol (data), then performs inverse Fourier transform, and since the sub-carriers are orthogonal to one another on the frequency axis, a symbol can be individually acquired for each sub-carrier by performing Fourier transform at the receive side.
OFDM Transmitter
FIG. 13 is a block diagram depicting an OFDM transmitter in a conventional OFDM radio communication system. An encoding section 1 encodes high-speed binary data using convolutional encoder or turbo encoder, and a modulation section 2 modulates the encoded data after interleave, using M-bits modulation such as BPSK, QPSK or 16-QAM, for example. Then a serial/parallel converter (S/P converter) 3 converts the modulated data symbol into N symbols of parallel modulated data symbols, and generates N number of sub-carrier components.
N points of inverted fast Fourier transform section 4 performs inverted Fourier transform processing (IFFT) on N number of modulated signals (sub-carrier components), which are output from the S/P converter 3, and outputs N number of time signal components in parallel. A parallel/serial converter (P/S converter) 5 converts the N number of time signal components acquired by the IFFT processing into serial data, and outputs them as an OFDM symbol. A guard interval insertion section 6 inserts a guard interval GI into this OFDM symbol, a digital/analog converter (D/A) converts the output signal from the guard interval insertion section into an analog signal, and a radio section 8 up-converts the frequency of the base band signal into a radio signal, then amplifies and transmits the radio signal into a space by an antenna 9. In the following description, a case of using fast Fourier transform FFT and inverted fast Fourier transform IFFT are used for Fourier transform and inverted Fourier transform will be described, but discrete Fourier transform DFT and inverted discrete Fourier transform IDFT may be used.
FIG. 14 is a diagram depicting a guard interval insertion. A guard interval insertion means that the end part of the OFDM symbol is copied and added to the beginning thereof. By inserting a guard interval GI, the influence of inter-signal interference ISI caused by multi-paths can be eliminated.
OFDM Receiver
FIG. 15 is a block diagram depicting an OFDM receiver in the OFDM radio communication system. A band pass filter (BPF) 11 performs filtering on a signal received by an antenna 10 to remove an unnecessary frequency component, a down-converter (D/C) 12 converts the frequency of a radio signal into a base band frequency, an analog/digital converter (not illustrated) converts the analog of the base band signal into digital, and a guard interval removal section 13 removes the guard interval. An S/P converter 14 converts a time signal, after the guard interval is removed, into N number of parallel data, and inputs the data to N points of the Fourier transform section 15. The Fourier transform section 15 performs N points of FFT processing on the N number of time signal components, and outputs N number of sub-carrier components. A channel estimation section (not illustrated) performs a known channel estimation operation and estimates channel coefficients of each sub-carrier, and generates a channel compensation value, and a channel compensation section 16 multiplies N number of FFT processing results by the channel compensation value so as to decrease the influence of channel distortion. Finally, a P/S converter 17 outputs N number of sub-carrier components after channel compensation is performed, sequentially in serial, a demodulation section 18 demodulates the input signal using BPSK, QPSK or 16-QAM, for example, and a decoding section 19 decodes and outputs the input data after deinterleave.
Prior Art
In radio communication, multi-paths, where a plurality of reflected waves reach the receiver via different routes, are a problem. If different reflected waves reach a receiver with certain time differences, inter-symbol interference occurs due to the overlap of time adjacent symbols, which deteriorates the bit error performance. In order to prevent this, inserting a guard interval is effective, which is not limited to OFDM. In the case of OFDM, as described in FIG. 14, it is common that the end part of the OFDM symbol is copied and added to the beginning thereof as a guard interval. If the maximum delay due to multi-paths is smaller than the guard interval length, inter-symbol interference due to multi-paths can be completely removed. In this case, as the guard interval length is longer, greater influence of the delay path can be removed, but transmission efficiency, that is, bit rate decreases. Therefore it is desirable to set the guard interval length to the length of the maximum path delay. However, in the case of applying the OFDM radio communication system to a cellular system, various cell arrangements and cell radiuses must be supported, and it is impossible to determine a guard interval length which is optimum for the entire system. Also, even within a cell, an optimum guard interval length differs since the distribution of multi-paths is different depending on the location of mobile stations.
Because of this, a radio communication system which can apply a plurality of guard interval lengths was proposed, and the adaptive control of the guard interval length, depending on the radio state, has been performed. A first prior art is a system where a base station determines a guard interval length and reports this guard interval length to mobile stations, and is disclosed in JP 2000-244441A, and JP 2001-69110A.
A second prior art is a system where a mobile station determines a guard interval length and reports this guard interval length to a base station, and the base station transmits a downlink signal using the guard interval length specified by the mobile station, and is disclosed in JP 2003-152670A, and JP 10-327122A.
A third prior art is a system where a mobile station detects a guard interval length in the blind (blind detection), and is disclosed in JP 2002-247005A and JP 2002-374223A.
Applying these prior arts to a radio communication system having scheduling and adaptive modulation functions will be considered. In such a radio communication system, a base station estimates a receive quality of each mobile station by some means, and decides the assignment of data transmission to each mobile station, the modulation system for data transmission and coding rate, for each packet timing based on the channel quality. According to this decision, the base station transmits a data packet and transmission control information including mobile station identification number, modulation system, coding rate, to the mobile station. The mobile station can accurately demodulate and decode data packets transmitted to itself by first receiving the transmission control information from the base station and then receiving the data signal using the transmission control information. For example, HSDPA (High-Speed Downlink Packet Access) in W-CDMA and HDR (High Data Rate) in CDMA 2000 use this system.
HSDPA System
FIG. 16 is a diagram depicting a configuration of an HSDPA system. BS is a base station, and UE#0 and UE#1 are mobile stations. In an HSDPA system, (1) HS-PDSCH (High-Speed-Physical Downlink Shared Channel) is used as a transmission channel of packet data in a downlink radio interval. This downlink data channel is shared by a plurality of mobile stations UE#0, UE#1, . . . . Also in the downlink radio interval, (2) HS-SCCH (High-Speed Shared Control Channel) is used as a high-speed control channel, and mobile stations UE#0 and UE#1 receives control information required for receiving packet data on HS-PDSCH. This HS-SCCH is shared by a plurality of mobile stations UE#0, UE#1 . . . . In the uplink radio channel, (3) HS-DPCCH (High-Speed Dedicated Physical Control Channel) is set for each user as a high-speed control channel for transmitting feedback information. In the HSDPA system, data retransmission control is performed between the base station BS and the mobile stations UE#0, UE#1, . . . , and the mobile stations UE#0, UE#1 . . . report ACK (acknowledgement) and NACK (not acknowledgement) depending on the decoding result of the received data to the base station BS using the above HS-DPCCH.
(A) to (D) of FIG. 17 are diagrams depicting the packet data receive mechanism of HS-PDSCH.
A transmission cycle called TTI (Transmission Time Interval=2 ms) is set on HS-SCCH, as shown in (A) of FIG. 17, and only when control information to transmit exists, this control information is transmitted from the base station BS at TTI, and is received by the mobile stations UE#0 and UE#1. The control data to be transmitted via HS-SCCH is, for example, a user identifier (UEID: User Equipment Identifier) and various parameters (e.g. spreading code, modulation scheme, data length) required for receiving data of HS-PDSCH.
The mobile stations UE#0 and UE#1 can receive HS-SCCH data at all the TTIs. For example, at slot #1 in (B) of FIG. 17, UE#0 and UE#1 receive HS-SCCH data simultaneously. Here mobile station UE#0 and UE#1 refer to the UEID in the data, and compares it with their own ID. In this case, the UEID of the HS-SCCH data in slot #1 is UE#1, so the mobile station UE#0 discards the received HS-SCCH data, and the mobile station UE#1 holds the control data in the received HS-SCCH data. Then the mobile station UE#1 extracts the parameter for receiving HS-PDSCH from the control data, and receives the packet data on HS-PDSCH ((C) and (D) of FIG. 17).
After receiving the data, the mobile station UE#1 refers to the CRC bit included in the data and judges whether the packet was successfully decoded without block error. If the data was received without block error, the mobile station UE#1 reports ACK to the base station BS using HS-DPCCH. If there is a block error, the mobile station UE#1 reports NACK to the base station BS using HS-DPCCH. This is the same for slots #2-5, and slots. #7-8, and the mobile station UE#1 receives the packet data via HS-PDSCH of slots #1 and 4, and the mobile station UE#0 receives packet data via HS-PDSCH of slots #2-3, 5 and 7-8.
Problems of Prior Art
If the first prior art is used for the radio communication system, the base station must report the guard interval length to the mobile station. However, the guard interval length, which is a basic parameter required for performing FFT for OFDM, must be known first to receive data. Therefore a method of reporting the guard interval length is a critical issue. Also, if a plurality of mobile stations are time-multiplexed as in the case of the above radio communication system, users are frequently switched, which means that the guard interval length must be frequently switched, and the mobile stations must judge the guard interval length for each packet. As a result, in each mobile station, a processing delay becomes a problem and the circuit size increases. In the base station, on the other hand, data transmission efficiency decreases because frequent transmission of information of the guard interval length is needed, and power consumption for transmitting control information increases.
If the second prior art is used for a radio communication system, a mobile station voluntarily determines a guard interval, and knows a guard interval before demodulation, so the problem of the first prior art does not exist. However, the reliability of the reported guard interval length from the mobile station to the base station must be increased, so highly efficient encoding is required for mobile stations, which increases the processing volume and power consumption, and makes the circuit complicated. Also the base station must adhere to the guard interval length specified by the mobile station, which is not desirable in terms of system management.
If the third prior art is used, highly reliable blind detection is required, which increases the processing volume of the mobile station and the power consumption, and requires a larger circuit scale accordingly. Also if a detection error is generated, the receive characteristic deteriorates, so it is difficult to implement this prior art.